A triac is a

A triac is a

Right Answer is:

3 terminal bidirectional switch


The major drawback of an SCR is that it can conduct current in one direction only. Therefore, an SCR can only control dc power or forward biased half-cycles of ac in a load. However, in an a.c system, it is often desirable and necessary to exercise control over both positive and negative half-cycles. For this purpose, a semiconductor device called triac is used.

The triac is a three-terminal ac switch that is triggered into conduction when a low-energy signal is applied to its gate terminal. Unlike the SCR, the triac conducts in either direction when turned on. The triac also differs from the SCR in that either a positive or negative gate signal trigger it into conduction. Thus the triac is a three-terminal, four layers, directional, semiconductor device that controls ac power whereas an SCR controls dc power or forward biased half cycles of ac in a load. Because of its bidirectional conduction property, the triac is widely used in the field of power electronics for control purposes.

“Triac” is an abbreviation for the three-terminal ac switch. ‘Tri’ indicates that the device has three terminals and ‘ac’ indicates the device controls alternating current or can conduct in either direction.


The triac is equivalent to two thyristors connected back to back with their gate terminals tied up. When MT2 is positive with respect to MT1, the SCR 1 is forward biased. If the gate is made positive with respect to MT2. the SCR 1 conducts and the device goes from high impedance state to low impedance state. When MT1 is made positive with respect to MT2, SCR 2 is forward biased and it conducts when the gate is made positive. Thus the triac can conduct in both directions. The SCR demands a positive voltage between the gate and cathode. But the triac can conduct with either positive or negative voltage at the gate.

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